Abusy signal (orbusy tone orengaged tone) intelephony is an audiblecall-progress tone or audiblesignal to thecalling party that indicates failure to complete the requestedconnection of that particulartelephone call.
The busy signal has become less common in the past few decades due to the prevalence ofcall waiting andvoicemail.
An otherwise unspecified busy signal indicates that the called number is occupied:
The standard busy signal sometimes occurs (sometimes with anintercept message played over the busy) at the end of a call toindicate the other party has hung up (seeDisconnect tone), but mostly theoff-hook tone is used. In some phone companies in the United Kingdom, the busy signal is played after thedial tone to indicate the caller has used up their allocated time to dial a number and must hang up, before the off-hook tone is played.
In the mid 1950s through the early 1980s, a telephone busy signal provided an early form ofsocial media in many cities and towns of the United States, colloquially known as the "beep line" or "jam line".[1][2] Due to a flaw in the telephone switching equipment, teenagers discovered they could talk to each other over the busy signal, often exchanging phone numbers, mostly for the purpose ofdating. Common phone numbers for this to form were on popularradio station request lines, where teens would be calling in en masse to try to win concert tickets or request their favorite songs, thus "jamming the lines" and generating a perpetual busy signal.
Areorder tone, sometimes called afast busy signal, indicates that notransmission path to the called number is available. This can occur either because the Inter-LATA trunk is busy at the time of the call- in which case this clears in a few seconds, if one redials- or the number is temporarily out of service, due to maintenance or the number is not willing to accept calls. It is otherwise played after a recorded announcement explaining the reason for a general call failure.
Countries have different signaling tones that act as busy signals, in most cases consisting of a tone with equal on/off periods at a rate of between 60 and 120 interruptions per minute (i.p.m.).
In North America, thePrecise Tone Plan used today employs two tones of 480 and 620Hz at an amplitude of -24 dBm with a 0.5 s on/off cadence. Prior to the adoption of the PreciseTone system, the busy signal generally had the same frequency as thedial tone. Until frequencies began to be standardized in the 1960s, telephone signals varied from telephone exchange to exchange.
TheEuropean Telecommunications Standards Institute (ETSI) recommendation for busy tone is a 425 Hz tone at -20dBm in a 0.5 s on/off cadence. This sequence was already in use in Portugal, Spain, The Netherlands, (West-)Germany, Italy, Greece, Switzerland, Lichtenstein, Iceland, Norway, Andorra, Monaco, San Marino, and Vatican City prior to publication of the ETSI recommendation.
The ETSI recommendation is now followed by all countries of the European Union. Historical oddities within the EU are:
The ETSI recommendation is also the default (i.e. non-localized) busy tone generated by mobile phones that follow theGSM & 3GPP family of standards.
The ETSI recommendation is also followed by someISDN equipment andPBX/office systems found outside Europe.
Most countries of the former Soviet Union, including Russia, are not members of the ETSI. These former Soviet republics employ a 425 Hz busy tone with a 0.4 s on/off cadence.
In the United Kingdom, the busy tone is a single 400 Hz tone with equal 0.375 s on/off periods. This was the case even when the UK was still part of the EU. The current 400 Hz/375ms tone was adopted in the mid-to-late 1960s and replaced the older busy tone, which was the same 400 Hz signal but at half the pulse duration, 0.75 s on, 0.75 s off.
